Planar lighting device

ABSTRACT

The planar lighting device includes a light guide plate having the light exit plane, two pairs of light entrance planes formed at the four sides of the light exit plane, and rear planes formed opposite to the light exit plane and inclined such that the light guide plate grows thicker toward the center thereof, a pair of main light sources and a pair of auxiliary light sources disposed opposite their respective light entrance planes to emit light to the respective light entrance planes, and illuminance distribution control unit to adjust the amount of light emitted by the main and auxiliary light sources to form a designated local illuminance distribution for any position desired in the light exit plane. The planar lighting device performs area control and line control to adjust illuminance at a light exit plane for any area desired and along any line desired, respectively.

This application is a divisional of U.S. application Ser. No.12/239,796, filed Sep. 28, 2008, which claims priority to JP2007-256027, filed Sep. 28, 2007, each of which is incorporated hereinby reference in its entirety.

The entire contents of literature cited in this specification areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a planar lighting device comprisinglight sources and alight guide plate for admitting light emitted by thelight sources and emitting the light through the light exit planethereof. The inventive planar lighting device is used for indoor andoutdoor illumination or as a backlight to illuminate the liquid crystaldisplay panel used in liquid crystal display devices or as a backlightused for advertising panels, advertising towers, advertising signs, andthe like.

Liquid crystal display devices use a backlight unit for radiating lightfrom behind the liquid crystal display panel to illuminate the liquidcrystal display panel. A backlight unit is configured using opticalmembers comprising a light guide plate for diffusing light emitted by anillumination light source to irradiate the liquid crystal display panel,a prism sheet, and a diffusion sheet.

Currently, large liquid crystal display televisions predominantly use aso-called direct illumination type backlight unit having no light guideplate but comprising optical members such as a diffusion plate disposedimmediately above the illumination light source. This type of backlightunit comprises cold cathode tubes serving as a light source provided onthe rear side of the liquid crystal display panel whereas the inside ofthe backlight unit provides white reflection surfaces to secure uniformlight amount distribution and a necessary brightness.

To achieve a uniform light amount distribution with the directillumination type backlight unit, however, the backlight unit needs tohave a given thickness, say about 30 mm, in a direction perpendicular tothe liquid crystal display panel. While demands of still thinnerbacklight units are expected to grow in the future, achieving a furtherreduced thickness of say 10 mm or less with a backlight unit isdifficult in view of uneven light amount distribution expected toaccompany the direct illumination type.

Among backlight units that allow reduction of thickness thereof is abacklight unit using a light guide plate whereby light emitted byillumination light sources and admitted into the light guide plate isguided in given directions and emitted through a light exit plane thatis different from the plane through which light is admitted.

There has been proposed a backlight of a type described above using alight guide plate formed by mixing scattering particles for diffusinglight into a transparent resin, for which reference may be had, forexample, to JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, and JP11-153963 A.

JP 07-36037 A, for example, discloses a light diffusion light guidelight source device comprising a light diffusion light guide memberhaving at least one light entrance plane region and at least one lightexit plane region and light source means for admitting light through thelight entrance plane region, the light diffusion light guide memberhaving a region that has a tendency to decrease in thickness with theincreasing distance from the light entrance plane.

JP 08-248233 A discloses a planar light source device comprising a lightdiffusion light guide member, a prism sheet provided on the side of thelight diffusion light guide member closer to a light exit plane, and areflector provided on the rear side of the light diffusion light guidemember. JP 08-271739 A discloses a liquid crystal display comprising alight emission direction correcting element formed of sheet opticalmaterials provided with a light entrance plane having a repeatedundulate pattern of prism arrays and a light exit plane given a lightdiffusing property. JP 11-153963 A discloses a light source devicecomprising a light diffusion light guide member having a scatteringpower therein and light supply means for supplying light through an endplane of the light diffusion light guide member.

In the planar lighting devices provided with a light diffusion lightguide plate containing light scatterers mixed therein as disclosed inthe above prior art literature, light emitted by the light source andadmitted through the light entrance plane into the light diffusion lightguide member receives a single or a multiple scattering effect at agiven rate as the light propagates through the inside of the lightdiffusion light guide member. Moreover, a significant proportion oflight that reaches both end planes of the diffusion light guide memberor a surface of the reflector receives reflection effect and is returnedback into the diffusion light guide member.

The above composite process produces light beam that is emitted throughthe light exit plane highly efficiently with a directivity to travelobliquely forward as viewed from the light source. Briefly, lightradiated by the light source is emitted through the light exit plane ofthe light diffusion light guide member.

Thus, the prior art literature mentioned above purportedly states that alight guide plate containing scattering particles mixed therein iscapable of emitting uniform light with a high light emission efficiency.

As regards the light guide plate used in the planar lighting device,there have been disclosed a light guide plate in the form of a flatplate and a light guide plate composed of a portion shaped to have aregion with a tendency to grow thinner with the increasing distance fromthe light entrance plane attached to the other portion, in addition tothe light guide plate mentioned above that is shaped to have a regionwith a tendency to grow thinner with the increasing distance from thelight entrance plane.

SUMMARY OF THE INVENTION

However, to achieve increased dimensions with a planar lighting deviceusing any of the light guide plates disclosed in the above prior artliterature, light needs to travel a longer distance from the lightsource, which in turn requires the light guide plate itself to be madethicker. Thus, an attempt to enlarge the display area of a planarlighting device is confronted with difficulties in reducing thethickness and the weight of the planar lighting device.

Further, a planar lighting device using a light guide plate having theshape that has a tendency to decrease in thickness with the increasingdistance from a position at which light from the light source isadmitted or the flat plate shape as disclosed in the prior artliterature mentioned above also oases a problem that a limited distancethat light is capable of traveling limits the extent to which thedimensions of the planar lighting device can be increased.

Further, conventional light guide plates generally used were incapableof locally adjusting the light intensity observed at the light exitplane for an area desired by adjusting the light intensities at thelight exit plane in the two mutually perpendicular directions thereof(referred to as “area control” below) or of adjusting the lightintensity observed at the light exit plane in a single directionperpendicular to the light entrance planes for a position (line) desiredperpendicular to a direction of movement (referred to as “line control”below). However, such an area control or line control, if made possible,provides a new mode of image display by adjusting the light intensityobserved at the light exit plane as desired and in various manners andnew applications for which planar lighting devices may be used.

It is an object of the present invention to solve the problemsassociated with the planar lighting devices disclosed in JP 07-36037 A,JP 08-248233 A, JP 08-271739 A, and JP 11-153963 A and provide a planarlighting device capable of area control whereby the light intensity atthe light exit plane is adjusted locally on an area by area basis orline control whereby the light intensity at the light exit plane isadjusted along any line desired, as described above.

To solve the above problems, the planar lighting device according to theinvention comprises a light guide plate including a light exit plane; apair of first light entrance planes formed respectively adjacent a pairof sides of the light exit plane; a pair of second light entrance planesformed respectively adjacent the other pair of sides of the light exitplane; and a pair of rear planes formed opposite to the light exit planeand inclined such that a thickness of the light guide plate in adirection perpendicular to the light exit plane increases with anincreasing distance from the pair of first light entrance planes,respectively, toward a central part of the light exit plane; a pair ofmain light sources disposed opposite the pair of first light entranceplanes of the light guide plate, respectively, and emitting light to thepair of first light entrance planes, respectively; a pair of auxiliarylight sources disposed opposite the pair of second light entrance planesof the light guide plate, respectively, and emitting light to the pairof second light entrance planes; and light intensity distributioncontrol means for adjusting amount of light emitted respectively by themain light sources and the auxiliary light sources to form a designatedlocal light intensity distribution for any position in the light exitplane of the light guide plate, wherein the main light sources and theauxiliary light sources each comprising light sources and a base onwhich the light sources are arrayed in a longitudinal direction of thepair of first light entrance planes and the pair of second lightentrance planes, respectively.

Preferably, the light intensity distribution control means comprises apattern memory for storing entered local light intensity distributionpatterns, a pattern reader for reading a designated local lightintensity distribution pattern from the pattern memory, and an LED drivefor outputting drive signals for the light source corresponding to thedesignated pattern.

Preferably, the light intensity distribution control means designates aposition in the light exit plane of the light guide plate by means of aposition in a direction parallel to one of the pairs of light entranceplanes and a position in a direction perpendicular to the direction anddesignates an amount of light emitted by each of the pair of main lightsources and an amount of light emitted by each of the pair of auxiliarylight sources thereby to control light intensity at any position in thelight exit plane of the light guide plate.

Preferably, the light intensity distribution control means comprises apattern memory for storing an entered intensity modulation line positionand an intensity modulation pattern, a position moving LED memory forreading a designated intensity modulation line position and a designatedintensity modulation pattern, and an LED drive for outputting drivesignals for the light sources corresponding to the line.

Preferably, amounts of the main light sources and the auxiliary lightsources are adjustable independently of each other and light intensityat the light exit plane of the light guide plate is adjusted accordingto signals from the light intensity distribution control means.

Preferably, the light guide plate contains numerous scattering particlestherein such that following inequalities hold:27/100000<(D2−D1)/(L/2)<26/1000 and0.08 Wt %<Np<0.25 Wt %.where Np is a density of the scattering particles, L a distance from thefirst light entrance plane to the second light entrance plane, D1 athickness of the light guide plate at the first light entrance planes,and D2 a thickness at a midpoint of the light guide plate.

Preferably, the light guide plate contains numerous scattering particlestherein such that following inequalities hold:1.1≦Φ·N _(p) ·L _(G) ·K _(C)≦8.20.005≦K_(C)≦0.1where Φ is a scattering cross section of the scattering particles, N_(p)a density of the scattering particles, K_(C) a compensation coefficient,and L_(G) a half of a length of the light guide plate in an optical axisdirection of the light guide plate.

According to the invention, the configuration as described above enablesefficient use of light emitted from the light source and emission oflight from the light exit plane free from unevenness in light intensity(brightness) or with reduced unevenness in light intensity (brightness),as well as area control whereby the amount of light at the light exitplane is adjusted locally on an area by area basis and line controlwhereby the light intensity at the light exit plane is adjusted alongany line desired.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will be apparent from the following detailed description andaccompanying drawings in which:

FIG. 1 is a schematic perspective view illustrating an embodiment of aliquid crystal display device using the planar lighting device of theinvention.

FIG. 2 is a cross sectional view of the liquid crystal display deviceillustrated in FIG. 1 taken along line II-II.

FIG. 3A is a view of an example of the planar lighting deviceillustrated in FIG. 2 taken along line III-III; FIG. 3B is a crosssectional view of FIG. 3A taken along line B-B.

FIG. 4A is a perspective view schematically illustrating a configurationof the light source used in the planar lighting device of FIGS. 1 and 2;FIG. 4B is a cross sectional view of the light source illustrated inFIG. 4A; and FIG. 4C is a schematic perspective view illustrating oneLED of the light source of FIG. 4A as enlarged.

FIG. 5 is a schematic perspective view illustrating a shape of theinventive light guide plate.

FIG. 6 is a graph illustrating measurements representing a relationshipbetween Φ·N_(p)·L_(G)·K_(C) and light use efficiency.

FIG. 7 is a graph illustrating measurements representing lightintensities of light emitted by light guide plates each having differentparticle densities.

FIG. 8 is a graph illustrating relationships between light useefficiency and light intensity unevenness on the one hand and particledensity on the other.

FIG. 9 is a graph illustrating a distribution of light intensitymeasured in a direction parallel to the plane of incidence and passingthrough the center of the light guide plate.

FIG. 10 is a graph illustrating an array density of LED chips in adirection parallel to the plane of incidence and passing through thecenter of the light guide plate.

FIG. 11 is a graph illustrating a light intensity distribution obtainedusing the LED chips arranged at the array density of FIG. 10 as measuredin a direction parallel to the plane of incidence and passing throughthe center of the light guide plate.

FIG. 12 is a graph illustrating a light intensity distribution obtainedusing the LED chips arranged at the array density of FIG. 10 as measuredin a direction parallel to the auxiliary light entrance planes of thelight guide plate and passing through the center thereof.

FIG. 13 is a view for explaining the area control.

FIG. 14 is a graph illustrating an example of light intensitydistribution curve as the area control is effected.

FIG. 15 is a graph illustrating another example of light intensitydistribution curve as the area control is effected.

FIG. 16 is a graph illustrating still another example of light intensitydistribution curve as the area control is effected.

FIG. 17 illustrates an example of light intensity distribution achievedby an area control.

FIG. 18 is a graph for explaining the line control.

FIG. 19 is a specific view for explaining the line control.

FIG. 20 illustrates an example of light intensity distribution achievedby the area control.

FIG. 21 illustrates another example of light intensity distributionachieved by the area control.

FIG. 22 illustrates still another example of light intensitydistribution achieved by the area control.

FIG. 23 is a block diagram illustrating an example of configuration of adrive unit for effecting the area control and the line control.

FIG. 24 is a graph illustrating how the light intensity varies in thedirection of the shorter side as the line control is effected.

FIG. 25 is a graph illustrating measurements of light use efficiencyobtained using light guide plates having different configurations.

FIG. 26 is a sectional view illustrating an example of a dual-layerlight guide plate.

FIG. 27 is a top plan view of an example where the optical axes of theauxiliary light sources are directed toward the center line.

DETAILED DESCRIPTION OF THE INVENTION

The planar lighting device of the invention will be described in detailbelow referring to an embodiment illustrated in the accompanyingdrawings. First, the basic configuration of the inventive planarlighting device will be described.

FIG. 1 is a schematic perspective view illustrating a liquid crystaldisplay device provided with the planar lighting device of theinvention; FIG. 2 is a cross sectional view of the liquid crystaldisplay device illustrated in FIG. 1 taken along line II-II.

FIG. 3A is a view of the planar lighting device (also referred to as“backlight unit” below) illustrated in FIG. 2 taken along line III-III;FIG. 3B is a sectional view of FIG. 3A taken along line B-B.

A liquid crystal display device 10 comprises a backlight unit 20, aliquid crystal display panel 12 disposed on the side of the backlightunit closer to the light exit plane, and a drive unit 14 for driving theliquid crystal display panel 12. In FIG. 1, part of the liquid crystaldisplay panel 12 is not shown to better illustrate the configuration ofthe planar lighting device.

In the liquid crystal display panel 12, electric field is partiallyapplied to liquid crystal molecules, previously arranged in a givendirection, to change the orientation of the molecules. The resultantchanges in refractive index in the liquid crystal cells are used todisplay characters, figures, images, etc., on the liquid crystal displaypanel 12.

The drive unit 14 applies a voltage to transparent electrodes in theliquid crystal display panel 12 to change the orientation of the liquidcrystal molecules, thereby controlling the transmittance of the lighttransmitted through the liquid crystal display panel 12.

The backlight unit 20 is a lighting device for illuminating the wholesurface of the liquid crystal display panel 12 from behind the liquidcrystal display panel 12 and comprises a light exit plane havingsubstantially a same shape as an image display surface of the liquidcrystal display panel 12.

As illustrated in FIGS. 1, 2, 3A and 3B, the backlight unit 20 accordingto the embodiment under discussion comprises a main body of the lightingdevice 24 and a housing 26. The main body of the lighting device 24comprises two main light sources 28, a light guide plate 30, an opticalmember unit 32, and a reflection plate 34. The housing 26 comprises alower housing 42, an upper housing 44, turnup members 46, and supportmembers 48. As illustrated in FIG. 1 (see also FIG. 2), a power supplycasing 49 is provided on the underside of the lower housing 42 of thehousing 26 to hold power supply units that supply the main light sources28 and the auxiliary light sources 29 with electrical power.

Now, components that make up the backlight unit 20 will be described.

The main body of the lighting device 24 comprises the main light sources28 for emitting light, the light guide plate 30 for admitting the lightemitted by the main light sources 28 to produce planar light, and theoptical member unit 32 for scattering and diffusing the light producedby the light guide plate 30 to obtain light with further reducedunevenness.

First, the main light sources 28 and the auxiliary light sources 29 willbe described.

The main light sources 28 and the auxiliary light sources 29 basicallyhave the same configuration except the position with respect to thelight guide plate 3C. Therefore, only the main light sources 28 will bedescribed as representative.

FIG. 4A is a perspective view schematically illustrating a configurationof the main light sources 28 of the planar lighting device 20 of FIGS. 1and 2; FIG. 4B is a cross sectional view of the main light source 28illustrated in FIG. 4A; and FIG. 40 is a schematic perspective viewillustrating a chip of only one light emitting diode (chip of a lightemitting diode being referred to as LED chip below) 50 of the main lightsource 28 of FIG. 4A as enlarged.

As illustrated in FIG. 4A, the main light source 28 comprises aplurality of LED chips 50 and a light source mount 52 as an example.

The LED chip 50 is a chip of a light emitting diode emitting blue lightthe surface of which has a fluorescent substance applied thereon. It hasa light emission face 58 with a given area through which white light isemitted.

Specifically, when blue light emitted through the surface of lightemitting diode of the LED chip 50 is transmitted through the fluorescentsubstance, the fluorescent substance generates fluorescence. Thus, whenblue light emitted by the LED chip 50 is transmitted through thefluorescent substance, the blue light emitted by the light emittingdiode and the light radiated as the fluorescent substance generatesfluorescence blend to produce and emit white light.

The LED chip 50 may for example be formed by applying a YAG (yttriumaluminum garnet) base fluorescent substance to the surface of a GaN baselight emitting diode, an InGaN base light emitting diode, etc.

As illustrated in FIG. 4B, the light source mount 52 comprises an arraybase 54 and fins 56. The LED chips 50 described above are arranged in asingle row on the array base 54 at given intervals. Specifically, theLED chips 50 are arrayed along the length of a first light entranceplane 30 d or a second light entrance plane 30 e of a light guide plate30 to be described, that is, parallel to a line in which the first lightentrance plane 30 d or the second light entrance plane 30 e meets with alight exit plane 30 a.

The array base 54 is a plate member disposed such that one surfacethereof faces the thinnest lateral end face of the light guide plate 30,i.e., the first light entrance plane 30 d or the second light entranceplane 30 e of the light guide plate 30. The LED chips 50 are carried ona lateral plane of the array base 54 facing the light entrance plane 30b of the light guide plate 30.

The array base 54 according to the embodiment under discussion is formedof a metal having a good heat conductance as exemplified by copper andaluminum. The array base 54 also acts as a heat sink to absorb heatgenerated by the LED chips 50 and releases the heat to the outside.

The fins 56 are plate members each formed of a metal having a good heatconductance as exemplified by copper and aluminum. The fins 56 areconnected to the array base 54 on the side thereof opposite from the LEEchips 50 and spaced a given distance from neighboring fins 56.

A plurality of fins 56 provided in the light source mount 52 ensure alarge surface area and a high heat dissipation efficiency, increasingthe efficiency with which the LED chips 50 are cooled.

The heat sink may be not only of air-cooled type but also ofwater-cooled type.

While the embodiment under discussion uses the array base 54 of thelight source mount 52 as heat sink, a plate member without aheat-releasing function may be used to form the array base in place ofthe array base having a function of a heat sink, where the LED chipsneed not be cooled.

As illustrated in FIG. 4C, the LED chips 50 of the embodiment underdiscussion each have a rectangular shape such that the sidesperpendicular to the direction in which the LED chips 50 are arrayed areshorter than the sides lying in the direction in which the LED chips 50are arrayed or in other words, the sides lying in the direction ofthickness of the light guide plate 30 to be described, i.e., thedirection perpendicular to the light exit plane 30 a, are the shortersides. Expressed otherwise, the LED chips 50 each have a shape definedby b>a where “a” denotes the length of the sides perpendicular to thelight exit plane 30 a of the light guide plate 30 and “b” denotes thelength of the sides in the array direction. Now, let “q” be the distanceby which the arrayed LED chips 50 are spaced apart from each other, thenq>b holds. Thus, the length “a” of the sides of the LED chips 50perpendicular to the light exit plane 30 a of the light guide plate 30,the length “b” of the sides in the array direction, and the distance “q”by which the arrayed LED chips 50 are spaced apart from each otherpreferably have a relationship satisfying q>b>a.

Providing the LED chips 50 each having the shape of a rectangle allows athinner design of the light source to be achieved while producing alarge amount of light. A thinner light source, in turn, enables athinner design of the planar lighting device to be achieved. Further,the number of LED chips that need to be arranged may be reduced.

While the LED chips 50 each preferably have a rectangular shape with theshorter sides lying in the direction of the thickness of the light guideplate 30 for a thinner design of the light source, the present inventionis not limited thereto, allowing the LED chips to have any shape asappropriate such as a square, a circle, a polygon, and an ellipse.

While the LED chips, arranged in a single row, has a monolayeredstructure in the embodiment under discussion, the present invention isnot limited thereto; one may use multilayered LED arrays for the lightsource comprising LED arrays each carrying LED chips 50 on the arraybase. Where the LEDs are thus stacked, more LED arrays can be stackedwhen the LED chips 50 are each adapted to have a rectangular shape andwhen the LED arrays are each adapted to have a reduced thickness. Wherethe LED arrays are stacked to form a multilayer structure, that is tosay, where more LED arrays (LED chips) are packed into a given space, alarge amount of light can be generated. Preferably, the above expressionalso applies to the distance separating the LED chips of an LED arrayfrom the LED chips of the LED arrays in adjacent layers. Expressedotherwise, the LED arrays preferably are stacked such that the LED chipsare spaced a given distance apart from the LED chips of the LED arraysin adjacent layers.

In the liquid crystal display device 10, provided with the main lightsources 28 and the auxiliary light sources 29 as in the example underdiscussion, the auxiliary light sources 29 are disposed opposite thefirst light entrance plane 30 f and the second light entrance plane 30 gof the light guide plate 30 thus providing four light entrance planes toadmit light also through the lateral sides of the light guide plate 30.Thus, the absolute value of the light intensity of light emitted throughthe light exit plane 30 a can be improved and the amount of overalllight provided by the liquid crystal display device 10 can be increased.

Further, the configuration where the light guide plate has the lightsources provided on all the four sides thereof allows a large amount ofillumination light to be emitted through the light exit plane, therebyachieving a larger display area for devices in which the light guideplate is used.

According to the invention, the amount of light emitted by the lightsources is preferably so adjusted that the light intensity distributionof the light emitted by the light guide plate represents a bell-curvedistribution. That is, a bell-curve distribution of light intensitysuitable for a display device such as a liquid crystal television isachieved by setting a different amount of light for each of the LEDchips constituting at least the main light sources.

In the main light sources 28 according to the embodiment underdiscussion, the LED chips 50 are independently set to their respectiveamounts of light such that the amount of light produced is greatestadjacent the central portion or area in the longitudinal direction ofthe first light entrance plane 30 d and the second light entrance plane30 e, decreasing with the increasing distance from the center towardboth ends.

The LED chips 50 are preferably set to their respective amounts of lightsuch that the light intensity of light as measured along the bisector aof the light guide plate 30 illustrated in FIG. 3A represents ahigh-in-the-middle, bell-curve distribution. Specifically, suppose thatthe amount of light of the LED chips opposite the central portion of thefirst light entrance plane 30 d and the second light entrance plane 30 eis 1, then the LED chips 50 are set to their respective amounts of lightsuch that an amount of light I satisfies 0<I≦1 in any other position.

Thus, the light intensity of the light emitted through the light exitplane 30 a can be formed into a bell-curve distribution, i.e., adistribution where the light intensity gradually increases toward thecenter, by independently setting the amount of each LED chip 50 suchthat the LED chips disposed opposite the central portion of the lightentrance plane produce a greater amount of light than those disposedopposite the periphery. Where the light emitted through the light exitplane exhibits a bell-curve light intensity distribution so as toprovide the central portion of the light entrance plane with the highestlight intensity, difference in light intensity between the central areaand the periphery appears to be evened out by visual observation suchthat uniform light seems to be emitted through the light exit plane.Thus, the planar lighting device of the invention is capable of emittinglight with a light intensity distribution that is suitable for use inliquid crystal televisions and the like.

To find such a desired light intensity distribution, one may for exampleuse a calculation based on a sequential iteration method or any otherappropriate known method.

Preferably, in the second example described above, the LED chips 50provided on the auxiliary light sources 29 are also independently set totheir respective amounts of light as are those on the main light sources28. That is, where a different amount of light is set independently foreach of the LED chips constituting the main light sources and theauxiliary light sources, a bell-curve brightness distribution suitablefor a display device such as a liquid crystal television can beachieved. Thus, different light intensities can be set in differentareas as designated in the light exit plane 30 a. In other words, theamount of light can be set two-dimensionally in the light exit plane 30a. Accordingly, an area control is made possible whereby the brightnessin the light exit plane can be adjusted from area to area.

The bell-curve light intensity distribution as measured on the lightexit plane can also be achieved with substantially the same effects byadjusting the array density of the LED chips otherwise than by settingthe amount of light of the LED chips independently. Specifically, theLED chips 50 a and 50 b provided on the main light sources 28 may bearranged in a row at an array density that varies according to theirposition along the length of the first light entrance plane 30 d and thesecond light entrance plane 30 e opposite which these LED chips aredisposed to achieve a bell-curve brightness distribution.

Further, since the LED chips 50 are so arranged that the LED chips awayfrom the center of the main light source 28 in the array direction maybe reduced in number, the manufacturing costs can be reduced and so canpower consumption as well.

Next, the light guide plate 30 will be described.

FIG. 5 is a perspective view schematically illustrating theconfiguration of the light guide plate 30.

As illustrated in FIGS. 2, 3, and 5, the light guide plate 30 comprisesthe light exit plane 30 a, which is flat and substantially rectangular;two light entrance planes, the first light entrance plane 30 d and thesecond light entrance plane 30 e, formed on both sides of the light exitplane 30 a and substantially perpendicular to the light exit plane 30 a;two inclined planes, a first inclined plane 30 b and a second inclinedplane 30 c, located on the opposite side from the light exit plane 30 a,i.e., on the underside of the light guide plate so as to be symmetricalto each other with respect to a central axis, or the bisector abisecting the light exit plane 30 a (see FIGS. 1 and 3) in a directionparallel to the first light entrance plane 30 d and the second lightentrance plane 30 e, and inclined a given angle with respect to thelight exit plane 30 a; and two lateral planes, a first lateral plane 30f and a second lateral plane 30 g, formed substantially vertical to thelight exit plane 30 a on the sides of the light exit plane 30 a on whichthe light entrance planes are not formed, i.e., on the two sidesperpendicular to the sides where the light exit plane 30 a and the lightentrance planes meet.

The first inclined plane 30 b and the second inclined plane 30 c are soinclined as to be distanced farther (spaced a longer distance) from thelight exit plane 30 a with the increasing distance from the first lightentrance plane 30 d and the second light entrance plane 30 e,respectively: expressed otherwise, the thickness of the light guideplate 30 in the direction perpendicular to the light exit plane 30 aincreases from the first light entrance plane 30 d and the second lightentrance plane 30 e toward the center of the light guide plate 30.

Thus, the light guide plate 30 is thinnest at both sides thereof, i.e.,at the first light entrance plane 30 d and the second light entranceplane 30 e, and thickest at the center, i.e., on the bisector α, wherethe first inclined plane 30 b and the second inclined plane 30 c meet.Expressed otherwise, the light guide plate 30 has such a configurationthat the thickness of the light guide plate 30 in the directionperpendicular to the light exit plane 30 a increases with the increasingdistance from the first light entrance plane 30 d or the second lightentrance plane 30 e. The inclination angle of the first inclined plane30 b and the second inclined plane 30 c with respect to the light exitplane 30 a is not specifically limited.

The two main light sources 28 mentioned above are disposed opposite thefirst light entrance plane 30 d and the second light entrance plane 30 eof the light guide plate 30, respectively. Specifically, one of the mainlight sources 28 comprising LED chips 50 a and a light source mount 52 ais disposed opposite the first light entrance plane 30 d and the othermain light source 28 comprising LED chips 50 b and a light source mount52 b is disposed opposite the second light entrance plane 30 e. In theembodiment under discussion, the light emission face 58 of the LED chips50 of the main light sources 28 has substantially the same length as thefirst light entrance plane 30 d and the second light entrance plane 30 ein the direction perpendicular to the light exit plane 30 a.

Thus, the planar lighting device 20 has the two main light sources 28disposed in such a manner as to sandwich the light guide plate 30. Inother words, the light guide plate 30 is placed between the two mainlight sources 28 arranged opposite each other with a given distancebetween them.

In the light guide plate 30 illustrated in FIG. 2, light entering thelight guide plate 30 through the first light entrance plane 30 d and thesecond light entrance plane 30 e is scattered as it travels through theinside of the light guide plate 30 by scatterers contained inside thelight guide plate 30 as will be described lacer in detail and, directlyor after being reflected by the first inclined plane 30 b or the secondinclined plane 30 c, exits through the light exit plane 30 a. Some lightcan in the process leak through the first inclined plane 30 b and thesecond inclined plane 30 c. However, it is then reflected by thereflection plate 34 provided on the side of the light guide plate closerto the first inclined plane 30 b and the second inclined plane 30 c toenter the light guide plate 30 again. The reflection plate 34 will bedescribed later in detail.

Likewise, the light emitted by the auxiliary light sources 29 andadmitted through the first auxiliary light entrance plane 30 f and thesecond auxiliary light entrance plane 30 g is scattered as it travelsthrough the inside of the light guide plate 30 by scatterers containedinside the light guide plate 30 as will be described later in detailand, directly or after being reflected by the first inclined plane 30 bor the second inclined plane 30 c, exits through the light exit plane 30a.

The shape of the light guide plate thus growing thicker in the directionperpendicular to the light exit plane 30 a with the increasing distancefrom the first light entrance plane 30 d or the second light entranceplane 30 e opposite which the main light source 28 is disposed allowsthe light admitted through the light entrance planes to travel fartherfrom the light entrance planes and, hence, enables a larger light exitplane to be achieved. Moreover, since the light entering through thelight entrance plane is advantageously guided to travel a long distancefrom the light entrance plane, a thinner design of the light guide plateis made possible.

The light guide plate 30 is formed of a transparent resin into whichscattering particles are kneaded and dispersed. Transparent resinmaterials that may be used to form the light guide plate 30 includeoptically transparent resins such as PET (polyethylene terephthalate),PP (polypropylene), PC (polycarbonate), PMMA (polymethyl methacrylate),benzyl methacrylate, MS resins, and COP (cycloolefin polymer). Thescattering particles kneaded and dispersed into the light guide plate 30may be formed, for example, of TOSPEARL (trademark), silicone, silica,zirconia, or a derivative polymer. The light guide plate 30 containingthe scattering particles is capable of emitting uniform illuminationlight through the light exit plane with a greatly reduced level ofunevenness in light intensity (brightness). The light guide plate 30 soformed may be manufactured using an extrusion molding method or aninjection molding method.

Now, let Φ be the scattering cross section of scattering particlescontained in the light guide plate 30; L_(G) the length in the incidentdirection from the first light entrance plane 30 d or the second lightentrance plane 30 e of the light guide plate 30 to a position where thethickness of the light guide plate 30 in the direction perpendicular tothe light exit plane 30 a is greatest, said incident direction,expressed otherwise, being the direction parallel to the direction inwhich light entering the light guide plate travels and perpendicular tothe line in which the light exit plane and the light entrance planes,i.e., the first light entrance plane and the second light entranceplane, meet, said length L_(G) being, in the embodiment underdiscussion, a half of the length of the light guide plate in theincident direction, which in the embodiment under discussion is thedirection perpendicular to the first light entrance plane 30 d of thelight guide plate 30, as also referred to as “direction of the opticalaxis” below, or, still otherwise expressed, the length from the firstlight entrance plane or the second light entrance plane to the bisectorα; N_(p) the density of the scattering particles contained in the lightguide plate 30, said density denoting the number of particles in unitvolume; and K_(C) a compensation coefficient. Then the valueΦ·N_(p)·L_(G)·K_(C) is preferably not less than 1.1 and not greater than8.2; the compensation coefficient K_(C) is preferably not less than0.005 and not greater than 0.1. The light guide plate 30, containingscattering particles satisfying the above relationship, is capable ofemitting uniform illumination light through the light exit plane 30 awith a greatly reduced level of brightness unevenness.

When parallel rays of light are caused to enter an isotropic medium, atransmittance T is generally expressed according to the Lambert-Beer lawby the following expression (1):T=I/I _(o)=exp(−ρ·x)  (1)

where x is a distance, I_(o) an intensity of incident light, I anintensity of outgoing light, and ρ an attenuation constant.

The above attenuation constant ρ can be expressed using the scatteringcross section of particles Φ and the number of particles N_(p) in unitvolume contained in the medium as follows:ρ=Φ·N _(p)  (2)

Accordingly, the light extraction efficiency E_(out) is expressed by thefollowing expression (3) where L_(G) is the length of the light guideplate in the direction parallel to the direction in which light enteringthe light guide plate travels from the light entrance planes of thelight guide plate as far as the thickest position or, in the embodimentunder discussion, a half of the length of the light guide plate in thedirection of the optical axis. Said half of the length of the lightguide plate in the direction of the optical axis denoted by L_(G) is thelength of the light guide plate 30 in the direction perpendicular to thelight entrance planes of the light guide plate 30 from one of the lightentrance planes of the light guide plate 30 to the center of the lightguide plate 30.

The light extraction efficiency E_(out) is a ratio of light reaching theposition spaced apart from the light entrance plane of the light guideplate by the length L_(G) in the direction of the optical axis to theincident light. In the case of the light guide plate 30 illustrated inFIG. 2, for example, the light extraction efficiency E_(out) is a ratioof light reaching the center of the light guide plate or, lighttraveling half the length of the light guide plate in the direction ofthe optical axis to the light incident on either end plane.E_(out)∝exp(−Φ·N_(p)·L_(G))  (3)

The expression (3) applies to a space of limited dimensions. Introducingthe compensation coefficient K_(C) therein to correct the relationshipwith the expression (I), the light extraction efficiency E_(out) isexpressed by the following expression (4). The compensation coefficientK_(C) is a dimensionless compensation coefficient empirically obtainedwhere light propagates through an optical medium of limited dimensions.E _(out)=exp(−Φ·N _(p) ·L _(G) ·K _(C))  (4)

According to the expression (4), when Φ·N_(p)·L_(G)·K_(C) is 3.5, thelight extraction efficiency E_(out) is 3%. When Φ·N_(p)·L_(G)·K_(C) is4.7, the light extraction efficiency E_(out) is 1%.

The results show that the light extraction efficiency E_(out) decreasesas Φ·N_(p)·L_(G)·K_(C) increases. The light extraction efficiencyE_(out) decreases in such a manner presumably because light is scatteredincreasingly as it travels in the direction of the optical axis of thelight guide plate.

It follows, therefore, that the greater the value Φ·N_(p)·L_(G)·K_(C)is, the more preferable it is as a property for the light guide plate.When Φ·N_(p)·L_(G)·K_(C) is great, light exiting through a planeopposite the light entrance plane can be reduced whereas light emittedthrough the light exit plane can be increased. Expressed otherwise, whenΦ·N_(p)·L_(G)·K_(C) is great, the ratio of light emitted through thelight exit plane to the light incident on the light entrance planes canbe increased. That ratio is also referred to as “light use efficiency”below. Specifically, a light use efficiency as high as 50% or more isachieved when Φ·N_(p)·L_(G)·K_(C) is 1.1 or greater.

While light emitted through the light exit plane 30 a of the light guideplate 30 increasingly exhibits light intensity unevenness asΦ·N_(p)·L_(G)·K_(C) increases, the light intensity unevenness can beheld to under a given, tolerable level by holding the valueΦ·N_(p)·L_(G)·K_(C) to 8.2 or less. Note that light intensity andbrightness can be treated substantially equally. Thus, it is assumedthat brightness and light intensity possess similar tendencies in thepresent invention.

Thus, the value Φ·N_(p)·L_(G)·K_(C) of the light guide plate 30 ispreferably not less than 1.1 and not greater than 8.2, and morepreferably not less than 2.0 and not greater than 8.0. Still morepreferably, the value Φ·N_(p)·L_(G)·K_(C) is not less than 3.0 and, mostpreferably, not less than 4.7.

The compensation coefficient K_(C) is preferably not less than 0.005 andnot greater than 0.1, thus 0.005≦K_(C)≦0.1.

Now, the light guide plate 30 will be described in greater detail byreferring to specific examples.

A computer simulation was conducted to obtain light use efficiencies fordifferent light guide plates given different values ofΦ·N_(p)·L_(G)·K_(C) by varying the scattering cross section Φ, theparticle density N_(p), the length L_(G), which is a half of the lengthof the light guide plate in the direction of the optical axis, and thecompensation coefficient K_(C). Further, light intensity unevenness wasevaluated. The light intensity unevenness (%) was defined as[(I_(Max)−I_(Min))/I_(Ave)]×100, where I_(Max) was a maximum lightintensity of light emitted through the light exit plane of the lightguide plate, I_(Min) a minimum light intensity, and I_(Ave) an averagelight intensity.

Because the incoming light emitted by the auxiliary light sources has aconstant light intensity irrespective of the position in the opticalaxis direction, the evaluation of the example was made under conditionswhere the auxiliary light sources are not provided.

The measurement results are shown in Table 1. In Table 1, judgments “O”indicate cases where the light use efficiency is 50% or more and thelight intensity unevenness is 150% or less whereas judgments “X”indicate cases where the light use efficiency is less than 50% or thelight intensity unevenness is more than 150%.

TABLE 1 N_(p) L_(G) Φ · N_(p) · Light use Light intensity Φ [m²][pcs/m³] [m] K_(C) L_(G) · K_(C) efficiency [%] unevenness [%] JudgmentExample 1 2.0 × 10⁻¹² 2.2 × 10¹⁴ 0.3 0.03 3.51 81.6 84 ◯ Example 2 2.0 ×10⁻¹² 4.3 × 10¹⁴ 0.3 0.02 6.21 84.7 149 ◯ Example 3 2.0 × 10⁻¹² 8.6 ×10¹⁴ 0.1 0.02 3.86 82.8 82 ◯ Example 4 1.1 × 10⁻¹⁰ 1.5 × 10¹³ 0.3 0.0083.91 83.0 105 ◯ Example 5 1.1 × 10⁻¹⁰ 2.0 × 10¹³ 0.3 0.007 4.98 84.3 142◯ Example 6 1.1 × 10⁻¹⁰ 3.5 × 10¹³ 0.1 0.007 2.86 79.2 47 ◯ Comparativeexample 1 2.0 × 10⁻¹² 2.2 × 10¹³ 0.3 0.05 0.66 29.1 51 X Comparativeexample 2 1.1 × 10⁻¹² 2.5 × 10¹² 0.3 0.01 0.99 43.4 59 X Comparativeexample 3 4.8 × 10⁻¹⁸ 8.6 × 10¹⁷ 0.1 15.2 6.26 84.8 201 X Comparativeexample 4 4.8 × 10⁻¹⁸ 1.7 × 10¹⁸ 0.1 13.9 11.5 84.9 225 X

FIG. 6 illustrates a relationship between Φ·N_(p)·L_(G)·K_(C) and lightuse efficiency, i.e., the ratio of light emitted through the light exitplane 30 a to light incident on the light entrance planes.

Table 1 and FIG. 8 show that given Φ·N_(p)·L_(G)·K_(C) of 1.1 or more, ahigh light use efficiency, specifically 50% or more, is achieved whereasgiven Φ·N_(p)·L_(G)·K_(C) of 8.2 or less, light intensity unevenness canbe held to 150% or less.

It is also shown that given K_(c) of 0.005 or more, a high light useefficiency is achieved, and given K_(c) of 0.1 or less, light intensityunevenness observed in light emitted from the light guide plate can bereduced to a low level.

Next, light guide plates varying in particle density N_(p) of theparticles kneaded or dispersed therein were fabricated to measurebrightness distributions of light emitted at different positions in thelight exit plane of the individual light guide plates. In the embodimentunder discussion, the conditions including scattering cross section Φ,length L_(G), which is a half of the length of the light guide plate inthe direction of its optical axis, compensation coefficient K_(C), andshape of the light guide plate, but excluding particle density N_(p),were respectively set to fixed values as the measurements were made. Inthe embodiment under discussion, therefore, the valueΦ·N_(p)·L_(G)·K_(C) changes in proportion as the particle density N_(p)changes.

FIG. 7 shows the measurements of the distribution of light intensityobserved in the light emitted through the light exit plane of theindividual light guide plates having different particle densities. FIG.7 shows the light intensity [lx] on the vertical axis plotted against alight guiding length, which is the distance [mm] from one of the lightentrance planes of the light guide plate on the horizontal axis.

Light intensity unevenness was calculated from[(I_(Max)−I_(Min))/I_(Ave)]×100 [%], where I_(Max) is a maximum lightintensity in the measured distribution of light emitted from areas ofthe light exit plane close to the lateral ends thereof, I_(Min) is aminimum light intensity, and I_(Ave) is an average light intensity.

FIG. 8 illustrates a relationship between the calculated light intensityunevenness and particle density. FIG. 8 shows the light intensityunevenness [%] on the vertical axis plotted against the particle density[pieces/m³] on the horizontal axis. Also shown in FIG. 10 is arelationship between light use efficiency and particle density, theparticle density being likewise indicated on the horizontal axis and thelight use efficiency [%] on the vertical axis.

As shown in FIGS. 7 and 8, increasing the particle density or,consequently, increasing Φ·N_(p)·L_(G)·K_(C), results in an enhancedlight use efficiency but then light intensity unevenness also increases.The graphs also show that reducing the particle density or,consequently, reducing Φ·N_(p)·L_(G)·K_(C), results in lowered light useefficiency but then light intensity unevenness decreases.

Φ·N_(p)·L_(G)·K_(C) of not less than 1.1 and not greater than 8.2 yieldsa light use efficiency of 50% or more and light intensity unevenness of150% or less. Light intensity unevenness, when reduced to 150% or less,is inconspicuous.

Thus, it will be understood that Φ·N_(p)·L_(G)·K_(C) of not less than1.1 and not greater than 8.2 yields light use efficiency above a certainlevel and a reduced light intensity unevenness.

According to the light guide plate 30 as described above used in theexample under discussion, the absolute value of the light intensityimproves by a factor of about 1.5 where the main light sources 28 andthe auxiliary light sources 29 are provided as compared with a casewhere only the main light sources 28 are provided.

Next, the optical member unit 32 will be described.

The optical member unit 32 serves to reduce the brightness unevenness ofthe illumination light emitted through the light exit plane 30 a of thelight guide plate 30 to achieve emission of light with reducedunevenness in light intensity (brightness) through a light emissionplane 24 a of the main body of the lighting device 24. As illustrated inFIG. 2, the optical member unit 32 comprises a diffusion sheet 32 a fordiffusing the illumination light emitted through the light exit plane 30a of the light guide plate 30 to reduce brightness unevenness, a prismsheet 32 b having micro prism arrays formed parallel to the lines wherethe light exit plane and the light entrance planes meet, and a diffusionsheet 32 c for diffusing the illumination light emitted through theprism sheet 32 b to reduce brightness unevenness.

The diffusion sheets 32 a and 32 c and the prism sheet 32 b may beprovided by making use, for example, of the diffusion sheets and theprism sheets disclosed in paragraphs [0028] through [0033] of JP2005-234397 A by the Applicant of the present application.

While the optical member unit in the embodiment under discussioncomprises the two diffusion sheets 32 a and 32 c and the prism sheet 32b between the two diffusion sheets, there is no specific limitation tothe order in which the prism sheet and the diffusion sheets are arrangedor the number thereof to be provided. Nor are the prism sheet and thediffusion sheets specifically limited, and use may be made of variousoptical members, provided that they are capable of reducing thebrightness unevenness of the illumination light emitted through thelight exit plane 30 a of the light guide plate 30.

For example, the optical members may also be formed of transmittanceadjusting members each comprising a number of transmittance adjustersconsisting of diffusion reflectors distributed according to thebrightness unevenness in addition to or in place of the diffusion sheetsand the prism sheet described above. Further, the optical member unitmay be adapted to have two layers formed using one sheet each of theprism sheet and the diffusion sheet or two diffusion sheets only.

Now, the reflection plate 34 of the main body of the lighting devicewill be described.

The reflection plate 34 is provided to reflect light leaking through thefirst inclined plane 30 b and the second inclined plane 30 c of thelight guide plate 30 back into the light guide plate 30 and helpsenhance the light use efficiency. The reflection plate 34 is shapedaccording to the contour of the first inclined plane 30 b and the secondinclined plane 30 c of the light guide plate 30 to cover the firstinclined plane 30 b and the second inclined plane 30 c. In theembodiment under discussion, the reflection plate 34 is shaped tocontour the sectionally triangular shape formed by the first inclinedplane 30 b and the second inclined plane 30 c as illustrated in FIG. 2.

The reflection plate 34 may be formed of any material as desired,provided that it is capable of reflecting light leaking through theinclined planes of the light guide plate 30. The reflection plate 34 maybe formed, for example, of a resin sheet produced by kneading, forexample, PET or PP (polypropylene) with a filler and then drawing theresultant mixture to form voids therein for increased reflectance; asheet with a specular surface formed by, for example, depositingaluminum vapor on the surface of a transparent or white resin sheet; ametal foil such as an aluminum foil or a resin sheet carrying a metalfoil; or a thin sheet metal having a sufficient reflective property onthe surface.

Upper light guide reflection plates 36 are disposed between the lightguide plate 30 and the diffusion sheet 32 a, i.e., on the side of thelight guide plate 30 closer to the light exit plane 30 a, covering themain light sources 28 and the end portions of the light exit plane 30 a,i.e., the end portion thereof closer to the first light entrance plane30 d and the end portion thereof closer to the second light entranceplane 30 e. Thus, the upper light guide reflection plates 36 aredisposed to cover an area extending from part of the light exit plane 30a of the light guide plate 30 to a part of the array bases 54 of themain light sources 28 in a direction parallel to the direction of theoptical axis. Briefly, two upper light guide reflection plates 36 aredisposed respectively on both end portions of the light guide plate 30.

The upper light guide reflection plates 36 thus provided prevents lightemitted by the main light sources 28 from leaking toward the light exitplane 30 a instead of entering the light guide plate 30.

Thus, light emitted from the LED chips 50 of the main light sources 28is efficiently admitted through the first light entrance plane 30 d andthe second light entrance plane 30 e of the light guide plate 30,increasing the light use efficiency.

The lower light guide reflection plates 38 are disposed on the side ofthe light guide plate 30 opposite from the light exit plane 30 a, i.e.,on the same side as the first inclined plane 30 b and the secondinclined plane 30 c, covering part of the main light sources 28. Theends of the lower light guide reflection plates 38 closer to the centerof the light guide plate 30 are connected to the reflection plate 34.

The upper light guide reflection plates 36 and the lower light guidereflection plates 38 may be formed of any of the above-mentionedmaterials used to form the reflection plate 34.

The lower light guide reflection plates 38 prevent light emitted by themain light sources 28 from leaking toward the first inclined plane 30 band the second inclined plane 30 c of the light guide plate 30 insteadof entering the light guide plate 30.

Thus, light emitted from the LED chips 50 of the main light sources 28is efficiently admitted through the first light entrance plane 30 d andthe second light entrance plane 30 e of the light guide plate 30,increasing the light use efficiency.

While the reflection plate 34 is connected to the lower light guidereflection plates 38 in the embodiment under discussion, theirconfiguration is not so limited; they may be formed of separatematerials.

The shapes and the widths of the upper light guide reflection plates 36and the lower light guide reflection plates 38 are not limitedspecifically, provided that light emitted by the main light sources 28is reflected and directed toward the first light entrance plane 30 d orthe second light entrance plane 30 e such that light emitted by the mainlight sources 28 can be admitted through the first light entrance plane30 d or the second light entrance plane 30 e and then guided toward thecenter of the light guide plate 30.

While, in the embodiment under discussion, the upper light guidereflection plates 36 are disposed between the light guide plate 30 andthe diffusion sheet 32 a, the location of the upper light guidereflection plates 36 is not so limited; it may be disposed between thesheets constituting the optical member unit 32 or between the opticalmember unit 32 and the upper housing 44.

Further, the upper light guide reflection plates 36 and the lower lightguide reflection plates 38 are preferably provided also at the ends ofthe first auxiliary light entrance plane 30 f and the second lightentrance plane 30 g of the light guide plate 30. Where the upper lightguide reflection plates 36 and the lower light guide reflection plates38 are provided also at the ends of the first auxiliary light entranceplane 30 f and the second light entrance plane 30 g of the light guideplate 30, the light emitted by the auxiliary light sources 29 can beefficiently admitted into the light guide plate.

Next, the housing 26 will be described.

As illustrated in FIGS. 1 to 3, the housing 26 accommodates and securestherein the main body of the lighting device 24 by holding it from aboveand both sides thereof, i.e., the light emission plane 24 a and thefirst inclined plane 30 b and the second inclined plane 30 c. Thehousing 26 comprises the lower housing 42, the upper housing 44, theturnup members 46, and the support members 48.

The lower housing 42 is open at the top and has a configurationcomprising a bottom section and lateral sections provided upright on thefour sides of the bottom section. Briefly, it has substantially theshape of a rectangular box open on one side. As illustrated in FIG. 2,the bottom section and the lateral sections support the main body of thelighting device 24 placed therein from above on the underside and on thelateral sides and covers the faces of the main body of the lightingdevice 24 except the light emission plane 24 a, i.e., the plane oppositefrom the light emission plane 24 a of the main body of the lightingdevice 24 (rear side) and the lateral sections.

The upper housing 44 has the shape of a rectangular box; it has anopening at the top smaller than the rectangular light emission plane 24a of the main body of the lighting device 24 and is open on the bottomside.

As illustrated in FIG. 2, the upper housing 44 is placed from above themain body of the lighting device 24 and the lower housing 42, that is,from the light exit plane side, to cover the main body of the lightingdevice 24 and the lower housing 42, which holds the former, as well asfour lateral sections 22 b.

The turnup members 46 have a substantially U-shaped sectional profilethat is identical throughout their length. That is, each turnup member46 is a bar-shaped member having a U-shaped profile in cross sectionperpendicular to the direction in which it extends.

As illustrated in FIG. 2, the turnup members 46 are fitted between thelateral sections of the lower housing 42 and the lateral sections of theupper housing 44 such that the outer face of one of the parallelsections of said U shape connects with lateral sections 22 b of thelower housing 42 whereas the outer face of the other parallel sectionconnects with the lateral sections of the upper housing 44.

To connect the lower housing 42 with the turnup members 46 and theturnup members 46 with the upper housing 44, one may use any knownmethod such as a method using bolts and nuts and a method using bonds.

Thus providing the turnup members 46 between the lower housing 42 andthe upper housing 44 increases the rigidity of the housing 26 andprevents the light guide plate from warping. As a result, for example,light can be efficiently emitted without, or with a greatly reducedlevel of, brightness unevenness. Further, even where the light guideplate used is liable to develop a warp, the warp can be corrected withan increased certainty or the warping of the light guide plate can beprevented with an increased certainty, thereby allowing light to beemitted through the light exit plane without brightness unevenness orwith a greatly reduced level of brightness unevenness.

While the upper housing, the lower housing and the turnup members of thehousing may be formed of various materials including metals and resins,lightweight, high-rigidity materials are preferable.

While the turnup members are discretely provided in the embodiment underdiscussion, they may be integrated with the upper housing or the lowerhousing. Alternatively, the configuration may be formed without theturnup members.

The support members 48 have an identical profile in cross sectionperpendicular to the direction in which they extend throughout theirlength. That is, each support member 48 is a bar-shaped member having anidentical cross section perpendicular to the direction in which itextends.

As illustrated in FIG. 2, the support members 48 are provided betweenthe reflection plate 34 and the lower housing 42, more specifically,between the reflection plate 34 and the lower housing 42 close to theend of the first inclined plane 30 b of the light guide plate 30 onwhich the first light entrance plane 30 d is located and close to theend of the second inclined plane 30 c of the light guide plate 30 onwhich the second light entrance plane 30 e is provided. The supportmembers 48 thus secure the light guide plate 30 and the reflection plate34 to the lower housing 42 and support them.

With the support members 48 supporting the reflection plate 34, thelight guide plate 30 and the reflection plate 34 can be brought into aclose contact. Furthermore, the light guide plate 30 and the reflectionplate 34 can be secured to a given position of the lower housing 42.

While the support members are discretely provided in the embodimentunder discussion, the invention is not limited thereto; they may beintegrated with the lower housing 42 or the reflection plate 34. To bemore specific, the lower housing 42 may be adapted to have projectionsto serve as support members or the reflection plates may be adapted tohave projections to serve as support members.

The locations of the support members are also not limited specificallyand they may be located anywhere between the reflection plate and thelower housing. To stably hold the light guide plate, the support membersare preferably located closer to the ends of the light guide plate or,in the embodiment under discussion, near the first light entrance plane30 d and the second light entrance plane 30 e.

The support members 48 may be given various shapes and formed of variousmaterials without specific limitations. For example, two or more of thesupport members may be provided at given intervals.

Further, the support members may have such a shape as to fill the spaceformed by the reflection plate and the lower housing. Specifically, thesupport members may have a shape such that the side thereof facing thereflection plate has a contour following the surface of the reflectionplate and the side thereof facing the lower housing has a contourfollowing the surface of the lower housing. Where the support membersare adapted to support the whole surface of the reflection plates,separation of the light guide plate and the reflection plate can bepositively prevented and, further, generation of brightness unevennessthat might otherwise be caused by light reflected by the reflectionplates can be prevented.

The planar lighting device 20 is basically configured as describedabove.

In the planar lighting device 20, light emitted by the main lightsources 28 provided on both sides of the light guide plate 30 strikesthe light entrance planes, i.e., the first light entrance plane 30 d andthe second light entrance plane 30 e, of the light guide plate 30 whilelight emitted by the auxiliary light sources 29 provided on the othertwo sides of the light guide plate 30 strikes the lateral planes, i.e.,the first lateral plane 30 f and the second lateral plane 30 g. Then,the light admitted through the respective planes is scattered byscatterers contained inside the light guide plate 30 as will bedescribed later in detail as the light travels through the inside of thelight guide plate 30 and, directly or after being reflected by the firstinclined plane 30 b or the second inclined plane 30 c, exits through thelight exit plane 30 a. In the process, part of the light leaking throughthe first inclined plane 30 b and the second inclined plane 30 c isreflected by the reflection plate 34 to enter the light guide plate 30again.

Thus, light emitted through the light exit plane 30 a of the light guideplate 30 is transmitted through the optical member 32 and emittedthrough the light emission plane 24 a of the main body of the lightingdevice 29 to illuminate the liquid crystal display panel 12.

The liquid crystal display panel 12 uses the drive unit 14 to controlthe transmittance of the light according to the position so as todisplay characters, figures, images, etc. on its surface.

The light intensity distribution of light emitted through the light exitplane was measured using the inventive planar lighting device.

In the example used for the measurement, the light guide plate had ashape as defined by the following dimensions: the length from the firstlateral plane 30 f to the second lateral plane 30 g measured 1000 mm; alength “d” of the first light entrance plane 30 d and the second lightentrance plane 30 e in the direction perpendicular to the light exitplane measured 580 mm; the length from the light exit plane 30 a to therear side at the bisector α, or, a maximum thickness D, measured 3.5 mm;and a length L_(G) from the first light entrance plane 30 d or thesecond light entrance plane 30 e to the bisector α measured 290 mm.

The weight ratio of the scattering particles mixed into the light guideplate to the light guide plate was 0.07 Wt %.

FIG. 9 illustrates light intensity distribution of light emitted throughthe light exit plane of the planar lighting device provided with thelight guide plate 30 having the above configuration where the lightintensity distribution illustrated was measured along the middle of thelight guide plate parallel to the first light entrance plane 30 d andthe second light entrance plane 30 e, i.e., on the bisector α.

FIG. 9 indicates relative light intensity [lx] on the vertical axisplotted against the position [mm] in the longitudinal direction of thefirst light entrance plane 30 d and the second light entrance plane 30 eof the light guide plate 30 given on the horizontal axis. The position“0” on the horizontal axis indicates the center of the light guide plate30 in the longitudinal direction of the first light entrance plane 30 dand the second light entrance plane 30 e; the positions “−500” and “500”indicate both ends of the light guide plate 30 in the longitudinaldirection of the first light entrance plane 30 d and the second lightentrance plane 30 e.

FIG. 10 illustrates an array density of the LED chips 50 varyingaccording to the position in the longitudinal direction of the firstlight entrance plane 30 d and the second light entrance plane 30 e.

FIG. 10 is a graph of the LED chip distribution illustrating the arraydensity of the LED chips 50 a and 50 b along the length of the mainlight sources 28 of the inventive planar lighting device 10.

FIG. 10 indicates on the horizontal axis the positions [sections] on thelight guide plate 30 each having a given unit length, which is 4 mm inthe example under discussion, into which the longitudinal length of thefirst light guide plate 30 d and the second light guide plate 30 e isdivided, plotted against the number of LED chips [pieces] arranged ineach section having a given unit length on the vertical axis. Thesection “0” on the horizontal axis indicates the 4-mm range containingthe center of the light guide plate 30 in the longitudinal direction ofthe first light entrance plane 30 d and the second light entrance plane30 e; the sections “−12” and “12” indicate the outermost ranges betweeneach of the outermost ends of the light guide plate 30 and a point 4 mmfrom said end toward the center in the longitudinal direction of thefirst light entrance plane 30 d and the second light entrance plane 30e.

The example under discussion uses two different array density patterns,I1 and X1, to array the LED chips 50 as illustrated in FIG. 10.

Generally, the array density is preferably so determined as to provide ahigh-in-the-middle, bell-curve light intensity distribution as measuredalong a bisector a of the light guide plate 30. To find an array densityfor the LED chips 50 whereby a bell-curve light intensity distributionis obtained, one may for example use a calculation based on a sequentialiteration method or any other appropriate known method.

In the example under discussion, the LED chips 50 are arranged at adensity of 14 pieces per section (4-mm range) in the array densitypatterns I1 and X1 in the positions opposite the central portion of thefirst light entrance plane 30 d and the second light entrance plane 30 eof the light guide plate 30, i.e., the position “0” on the horizontalaxis, as illustrated in FIG. 10. The array density of the LED chips 50decreases with the increasing distance from the center of the firstlight entrance plane 30 d and the second light entrance plane 30 e Suchthat the number of LED chips 50 per section is 0 in the positions “−11”and “11” in the case of array density pattern I1 and in the positions“−12” and “12” in the case of array density pattern X1, i.e., thepositions opposite both ends of the first light entrance plane 30 d andthe second light entrance plane 30 e.

In both patterns I1 and X1, the array density peaks adjacent the centralportion of the first light entrance plane 30 d and the second lightentrance plane 30 e, i.e. adjacent the position “0” on the horizontalaxis in FIG. 10. Now, let the array density be 1 at the center, then theLED chips 50 are arrayed at an array density D satisfying 0<D≦1 in anyother position.

The LED chips 50 are preferably arranged such that the array density ishighest at a central portion in the lengthwise direction of the firstlight entrance plane 30 d and the second light entrance plane 30 e, thearray density decreasing with the increasing distance from the center.Thus, a bell-curve distribution can be obtained for the light emitted bythe light guide plate 30.

FIGS. 11 and 12 illustrate light intensity distributions obtained usingthe light guide plate 30 where the LED chips are arrayed at the densitygiven in FIG. 10.

FIG. 11 illustrates light intensity distributions of the light asmeasured on the light exit plane 30 a in a direction parallel to thelongitudinal direction of the first light entrance plane 30 d and thesecond light entrance plane 30 e and passing through the center of thelight guide plate 30. In this case, the LED chips 50 were arranged atthe array density patterns I1 and X1 of FIG. 10. FIG. 11 indicates therelative light intensity [lx] on the vertical axis plotted against theposition [mm] in the longitudinal direction of the first light entranceplane 30 d and the second light entrance plane 30 e of the light guideplate 30 on the horizontal axis.

As illustrated in FIG. 11, whether the array density pattern I1 or X1 isused, the light intensity peaks adjacent the central area, i.e.,adjacent the position “0” on the horizontal axis and decreases down theperiphery, representing a bell-curve light intensity distribution.

FIG. 12 illustrates a light intensity distribution of light on the lightexit plane 30 a in a direction parallel to the longitudinal direction ofthe first auxiliary light entrance plane 30 f and the second auxiliarylight entrance plane 30 g and passing through the center of the lightguide plate 30. Note that in the example under discussion, the arraydensity of the LED chips 50 is constant throughout the length of theauxiliary light sources 29.

FIG. 12 indicates the relative light intensity [lx] on the vertical axisplotted against the position [mm] in the longitudinal direction of thefirst auxiliary light entrance plane 30 f and the second auxiliary lightentrance plane 30 g on the horizontal axis.

As is apparent from FIG. 12, the light intensity distribution along thelength of the first auxiliary light entrance plane 30 f and the secondauxiliary light entrance plane 30 g represents a bell curve distributionwhere the light intensity peaks at the center, i.e., adjacent theposition “0” on the horizontal axis, decreasing with the increasingdistance from the center toward both ends, almost regardless of whetherthe LED chips 50 on the main light sources 28 are arrayed at the arraydensity pattern I1 or X1.

It follows, therefore, that where the LED chips 50 are provided on themain light sources 28 with such array densities as illustrated in FIG.10, light intensity distributions as illustrated in FIGS. 11 and 12 canbe obtained where the light intensity adjacent the central portion ofthe light exit plane 30 a is higher than in the periphery thereof,representing a high-in-the-middle, bell-curve light intensitydistribution.

While it is generally preferable that, as described above, the arraydensity is so determined such that the light intensity distribution overthe light guide plate 30 represents a bell-curve distribution (ahigh-in-the-middle distribution) as seen two-dimensionally, there arecases where light intensity distribution characteristics different fromsuch a distribution may be preferred depending upon the image to bedisplayed, for example light intensity distribution characteristicslocally exhibiting remarkable differences. Light intensity distributionsin such cases are particularly effective where, for example, one desiresto provide a display purposely using a special light intensitydistribution.

Thus, the inventive planar lighting device 10 provides two differentlight intensity distribution adjusting means to meet requirements ofsuch displays as well. The two different light intensity distributionadjusting means use the area control and the line control describedearlier, respectively.

A first light intensity distribution adjusting means, the area controlmeans, will be described first.

According to the basics of the area control, the light exit plane of theplanar lighting device is divided vertically and horizontally to provideareas as illustrated in FIG. 13, for example (divided in a proportion of0.75:1:0.75 each representing a relative light intensity in the examplegiven here), adjusting the light intensities of the LEDs in thedirection of the shorter side of the light exit plane (the verticaldirection in FIG. 13) or in the direction of the longer side (thehorizontal direction in FIG. 13) to provide different light intensitydistributions as illustrated in FIGS. 14A and 14B, respectively.

In FIG. 13, the relative light intensities “0,” “0,” and “0” for thethree sections into which the shorter side of the light exit plane isdivided indicate that no light is admitted in this direction.

In FIG. 13, the areas defined by dividing the light exit plane into fivesections both vertically and horizontally represent photometric areasfor the photoreceivers used. Thus, the difference between the areadivision for setting a light intensity distribution and the areadivision for photometry does not pose a substantial problem.

The light intensity distributions illustrated in FIGS. 14A and 14B areexamples of a so-called high-in-the-middle curve and other lightintensity distributions may of course be used as desired. FIGS. 14A and14B represent light intensity distributions as measured in thedirections of the shorter side and the longer side, respectively.

Where the display intended is rectangular, for example, use may be madeof an a light intensity distribution inclined in the direction of theshorter side (see FIG. 15A), a light intensity distribution inclined inthe direction of the longer side (see FIG. 15B), and a light intensitydistribution inclined in the direction of the diagonal (see FIGS. 16Aand 16B).

Note that the light intensity distribution in the direction of theshorter side representing a smooth curve (see FIG. 14A) and the lightintensity distribution in the direction of the longer side representinga polygonal line (see FIG. 14B) are merely examples; various lightintensity distribution characteristics may of course be selected anddesignated in the directions of the shorter and longer sides and fordesired positions in each of these directions.

FIG. 17 illustrates an example of a three-dimensional representation ofthe light intensity distribution for a displayed image where theadjustments of the light intensity distributions as illustrated in FIGS.14A and 14B are made.

In the example of FIG. 17, the aspect of the high-in-the-middle(bell-curve) light intensity distribution peaking near the middle bothin the directions of the shorter and the longer sides is representedwith virtual cross-sections taken both in the directions of the shorterand the longer sides.

Adjustment of the light intensity distribution to provide desired lightintensity distributions in the directions of the shorter sides and thelonger sides as described above may be preferably achieved, for example,by storing LED illumination distribution patterns in a memory providedin, for example, the drive unit 14 that drives the above-mentionedliquid crystal display panel 12, selecting and reading a designatedpattern from among these patterns, and outputting an LED drive signalbased upon the information thus given into an LED drive from lightintensity distribution control means provided in, for example, the driveunit 14, thereby to control the LED light intensities accordingly.

Next, a second light intensity distribution adjusting means, the linecontrol means, will be described.

FIG. 18 illustrates a range of a contrast, which is a maximum opticalintensity divided by a minimum optical intensity, observed as a linearoptical intensity modulation is effected individually for positionsdesired in the screen.

According to the basics of the line control, the LED light intensityadjustments are so effected as to permit moving the data to be displayedby LEDs, of which the positions are predetermined, to any positionswithin an allowable range respectively along one of the shorter andlonger sides of the light exit plane of the planar lighting device asillustrated in FIG. 19 to achieve a display as intended (descriptionhere is made taking the longer side by way of example).

In the example illustrated in FIG. 19, the display positions or thepositions of the LEDs of which the light intensity is adjusted are movedby the line control means such that 76 LEDs located near the center aredesignated from among the LEDs disposed along the longer side, and theposition where the data corresponding to the image to be displayed inthe area occupied by those 76 LEDs is to be displayed is freely moved bya distance not exceeding 314 mm, in this example, to achieve a displayintended.

FIGS. 20A, 20B, 21A, 21B, and 22 illustrate respectively, by way ofexample, how the light intensity distribution appears as the distance ofmovement, or displacement, from a reference point in a given directionis changed. Note that the five drawings illustrate a series of movementalthough they are divided into three sheets of drawings because of spacelimitations.

FIGS. 20A to 22 illustrate light intensity distributions withdisplacements of 0 mm, 20 mm, 40 mm, 60 mm and 80 mm, respectively.

While, in the examples illustrated in FIGS. 20A to 22, the dimension inthe movement direction gradually changes (grows larger) as thedisplacement increases, the width of a linear intensity distribution maybe matched for all the positions by changing the optical emissionintensity distribution for any position desired along the LED array.

FIG. 23 illustrates an example of drive unit configuration forperforming the area control and the line control described above. In theexample illustrated in FIG. 23, the drive unit 14 for driving the liquidcrystal display panel 12 comprises an area control unit 14A and a linecontrol unit 14B. In response to an instruction, the area control unit14A or the line control unit 14B, whichever is given the instruction,reads out stored data, which is an area control pattern or a linecontrol displacement, and supplies it to the LED drive.

Now, in the case of the control by the area control unit 14A, either adesired pattern is designated from a pattern memory that storesillumination distribution patterns within the light exit plane (for theshorter and longer sides) or a desired pattern is entered from theoutside to read out the pattern and give an instruction to the LED driveaccordingly.

In the case of the control effected by the line control unit 14B, sincethe position movement is effected only in the direction of the longerside in the example under discussion, and hence the positions of theLEDs to be driven are changed (see FIG. 19) only in that direction, theline control can be achieved easily by giving instructions for changingthe positions (numbers) of the LEDs to be driven according to theirrespective displacements.

FIG. 24 illustrates how the light intensities change in the direction ofthe shorter side (widthwise direction) as the movement described aboveis effected. As is apparent from this graph, the displacement littleaffects the light intensity observed in the direction of the shorterside (widthwise direction), making it possible to move the displaypositions by a unit of an area having a given width in the direction ofthe longer side (movement direction), i.e., line by line.

In other words, a linear optical intensity modulation can be effectedlocally according to the superposition principle.

According to the above embodiment, display of an image using a locallight intensity distribution that is different from a standard lightintensity distribution is easily achieved by selecting and using eitherthe area control, which is the first light intensity distributionadjusting means, or the line control, which is the second lightintensity distribution adjusting means. Further, display purposely usinga special light intensity distribution may also be readily achieved.

Further, in the light guide plate of the planar lighting deviceaccording to the embodiment under discussion, the embodiment underdiscussion, let D1 be the thickness of the light guide plate at itslight entrance plane (thickness of the light guide plate at a locationat which light is admitted); D2 the thickness of the light guide plateat a location where the light guide plate is thickest, which, in theembodiment under discussion, is the thickness of the light guide platewhere the bisector a thereof is located (thickness at the center); and Lthe length of the light guide plate in the incident direction from thefirst light entrance plane to the second light entrance plane (lightguiding length), L being 2L_(G) in the embodiment under discussion.Then, it is preferable that the following relationships hold:D1<D2 and27/100000<[(D2−D1)/(L/2)]<5/100; andthat the ratio Npa of the weight of the scattering particles containedto the weight of the light guide plate satisfies a range:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency of the main light sources can be increased to 30% or more.

Alternatively, it is also preferable that the light guide plate isimproved such that the following relationships hold:D1<D2 and66/100000<[(D2−D1)/(L/2)]<26/1000; andthat the ratio Npa of the weight of the contained scattering particlesto the weight of the light guide plate satisfies a range:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency of the main light sources can be increased to 40% or more.

It is preferable that the light guide plate is improved such that thefollowing relationships hold:D1<D2 and1/1000<[(D2−D1)/(L/2)]<26/1000; andthat the ratio Npa of the weight of the contained scattering particlesto the weight of the light guide plate satisfies a range of:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency can be increased to 50% or more.

FIG. 25 illustrates measurements of the light use efficiency of lightguide plates of which the inclined planes have different inclinationangles from each other, i.e., light guide plates having various shapeswith different values of (D2−D1)/(L/2). Because the light guide platesaccording to the example used for the measurements have a flat shape inthe direction in which light emitted by the auxiliary light sourcestravels and, therefore, light use efficiency is substantially notchanged by the shape of the inclined planes, only the main light sourceswere provided to measure the light use efficiency thereof.

FIG. 25 indicates [(D2−D1)/(L/2)] of the light guide plate on thehorizontal axis plotted against light use efficiency [%] on the verticalaxis.

As will be apparent from the measurements illustrated in FIG. 25, whenthe light guide plate has a shape satisfying27/100000<[(D2−D1)/(L/2)]<5/100, the light use efficiency can beincreased to 30 or more; when the light guide plate has a shapesatisfying 66/100000<[(D2−D1)/(L/2)]<26/1000, the light use efficiencycan be increased to 40% or more; and when the light guide plate has ashape satisfying 1/1000<[(D2−D1)/(L/2)]<26/1000, the light useefficiency can be increased to 50% or more.

Further, it is also preferable to use a multilayered light guide plate30 as illustrated in FIG. 26 composed of different materials containingscattering particles in different ratios by weight Npa. FIG. 26illustrates an example of light guide plate composed of two differentmaterials 30A and 30B.

Where the two materials each contain the scattering particles in anappropriate ratio by weight Npa with respect to each other, excellentoptical scattering characteristics may be achieved that are unobtainableusing materials all containing scattering particles in an identicalratio.

Further, where the light guide plate is rectangular, directing theoptical axes of the LED chips 50 provided on the shorter sides to formthe auxiliary light sources 29 toward the center line of the light guideplate as illustrated in FIG. 27 is also effective to make adjustments orachieving a high-in-the-middle, bell-curve light intensity distribution.In the example shown, the LEDs located at the extreme ends of botharrays of auxiliary light sources are directed toward the center of theopposite array of auxiliary light sources (intersection with the centerline α), with the other LEDs directed parallel to the LEDs located onthe extreme ends of the LED array. The LEDs may be directed in any otherappropriate manner as desired, however.

The light guide plate may have any shape without limitation to the aboveshape, provided that the thickness of the light guide plate increaseswith the increasing distance from the light entrance planes.

For example, fine prism arrays may be formed on the first inclined plane30 b and the second inclined plane 30 c in the direction parallel to thefirst light entrance plane 30 d and the second light entrance plane 30e. Instead of such prism arrays, optical elements similar to prisms maybe provided and arranged regularly. For example, elements having lenseffects such as lenticular lenses, concave lenses, convex lenses, oroptical elements in pyramidal shape may be formed on the inclined planesof the light guide plate.

While the inventive planar lighting device has been described above indetail, the invention is not limited in any manner to the aboveembodiment and various improvements and modifications may be madewithout departing from the spirit of the present invention.

For example, in the inventive planar lighting device, while each LEDchip of the light sources is formed by applying YAG fluorescentsubstance to the light emission face of a blue LED, the LED chip may beformed otherwise without limitations to such a configuration. Forexample, the LED chip used herein may be formed using a differentmonochromatic LED such as a red LED or a green LED with a fluorescentsubstance.

Further, an LED unit formed using three kinds of LEDs, i.e., a red LED,a green LED, and a blue LED, may be used in that case, light beamsemitted by the three kinds of LEDs are blended to produce white light.

Alternatively, a semiconductor laser (LD) may be used instead of an LED.

What is claimed is:
 1. A planar lighting device comprising: a lightguide plate including: a light exit plane; a pair of first lightentrance planes formed respectively adjacent a pair of sides of saidlight exit plane; a pair of second light entrance planes formedrespectively adjacent the other pair of sides of said light exit plane;and a rear plane formed opposite to said light exit plane; a pair ofmain light sources disposed opposite said pair of first light entranceplanes of said light guide plate, respectively, and emitting light tosaid pair of first light entrance planes, respectively; a pair ofauxiliary light sources disposed opposite said pair of second lightentrance planes of said light guide plate, respectively, and emittinglight to said pair of second light entrance planes; and light intensitydistribution control means for adjusting amount of light emittedrespectively by said main light sources and said auxiliary light sourcesto form a designated local light intensity distribution for any positionin said light exit plane of said light guide plate, wherein said mainlight sources and said auxiliary light sources each comprising lightsources and a base on which said light sources are arrayed in alongitudinal direction of said pair of first light entrance planes andsaid pair of second light entrance planes, respectively.
 2. The planarlighting device according to claim 1, wherein said light intensitydistribution control means comprises a pattern memory for storingentered local light intensity distribution patterns, a pattern readerfor reading a designated local light intensity distribution pattern fromsaid pattern memory, and an LED drive for outputting drive signals forsaid light source corresponding to said designated pattern.
 3. Theplanar lighting device according to claim 1, wherein said lightintensity distribution control means designates a position in said lightexit plane of said light guide plate by means of a position in adirection parallel to one of said pairs of light entrance planes and aposition in a direction perpendicular to said direction and designatesan amount of light emitted by each of said pair of main light sourcesand an amount of light emitted by each of said pair of auxiliary lightsources thereby to control light intensity at any position in said lightexit plane of said light guide plate.
 4. The planar lighting deviceaccording to claim 1, wherein said light intensity distribution controlmeans comprises a pattern memory for storing an entered intensitymodulation line position and an intensity modulation pattern, a positionmoving LED memory for reading a designated intensity modulation lineposition and a designated intensity modulation pattern, and an LED drivefor outputting drive signals for said light sources corresponding tosaid line.
 5. The planar lighting device according to claim 1, whereinamounts of said main light sources and said auxiliary light sources areadjustable independently of each other and wherein light intensity atsaid light exit plane of said light guide plate is adjusted according tosignals from said light intensity distribution control means.
 6. Theplanar lighting device according to claim 1, wherein said light guideplate contains numerous scattering particles therein such that followinginequalities hold:27/100000<(D2−D1)/(L/2)<26/1000 and0.08 Wt %<Np<0.25 Wt % where Np is a density of said scatteringparticles, L a distance from said first light entrance plane to saidsecond light entrance plane, D1 a thickness of the light guide plate atsaid first light entrance planes, and D2 a thickness at a midpoint ofsaid light guide plate.
 7. The planar lighting device according to claim1, wherein said light guide plate contains numerous scattering particlestherein such that following inequalities hold:1.1≦Φ·N _(p) ·L _(G) ·K _(c)≦8.20.005≦K_(c)≦0.1 where Φ is a scattering cross section of said scatteringparticles, N_(p) a density of said scattering particles, K_(c) acompensation coefficient, and L_(G) a half of a length of said lightguide plate in an optical axis direction of said light guide plate.
 8. Aplanar lighting device comprising: a light guide plate including: alight exit plane; a first light entrance plane formed adjacent a side ofsaid light exit plane; a second light entrance plane formed adjacent theother pair of sides of said light exit plane; and a rear plane formedopposite to said light exit plane; a pair of main light sources disposedopposite said first light entrance plane of said light guide plate, andemitting light to said first light entrance plane; a pair of auxiliarylight sources disposed opposite said second light entrance plane of saidlight guide plate and emitting light to said second light entranceplane; and light intensity distribution control means for adjustingamount of light emitted respectively by said main light sources and saidauxiliary light sources to form a designated local light intensitydistribution for any position in said light exit plane of said lightguide plate, wherein said main light source and said auxiliary lightsource each comprising light sources and a base on which said lightsources are arrayed in a longitudinal direction of said first lightentrance plane and said second light entrance plane.
 9. The planarlighting device according to claim 1, further comprising an imagedisplaying means for displaying an image on its surface by controllinglight intensity by said light intensity distribution control means,wherein said pattern reader reads a local light intensity distributionpattern designated in accordance with said image displayed on said imagedisplay means from said pattern memory.
 10. The planar lighting deviceaccording to claim 8, further comprising an image displaying means fordisplaying an image on its surface by controlling light intensity bysaid light intensity distribution control means, wherein said lightintensity distribution control means comprises a pattern memory forstoring entered local light intensity distribution patterns, a patternreader for reading a local light intensity distribution pattern fromsaid pattern memory, the local light intensity distribution patternbeing designated in accordance with said image displayed on said imagedisplay means, and an LED drive for outputting drive signals for saidmain light source and said auxiliary light source corresponding to saiddesignated pattern.
 11. The planar lighting device according to claim 8,wherein said light intensity distribution control means designates aposition in said light exit plane of said light guide plate by means ofa position in a direction parallel to one of said first light entranceplane and said second light entrance plane and a position in a directionperpendicular to said direction and designates an amount of lightemitted in each of said directions by said main light source and anamount of light emitted in each of said directions by said auxiliarylight source to thereby control light intensity at said designatedposition in said light exit plane of said light guide plate.
 12. Theplanar lighting device according to claim 8, wherein said main lightsource and said auxiliary light source comprise LEDs, wherein said lightintensity distribution control means comprises: a position moving LEDmemory for storing positions of predetermined LEDs among said LEDs ofsaid main light source or said auxiliary light source and an intensitymodulation pattern displayed by said predetermined LEDs; a LED positionreader for reading said positions of predetermined LEDs and saidintensity modulation pattern specified based on display image data fromsaid LED memory; and a LED drive for moving said LEDs along said firstlight entrance plane or said second light entrance plane based on saidpositions of predetermined LEDs and said display image data suppliedfrom said LED position reader and driving said LEDs based on saidintensity modulation pattern supplied from said LED position reader.